Propylene and butene are critically important chemical raw materials, and the global industrial demand for these two olefins is enormous. Along with the rapid development of downstream products, the gap between supply and demand widens year by year, and their prices have been rising uninterruptedly, far higher than the price of ethylene. Nowadays, propylene and butene mainly come from steam cracking and catalytic cracking of oil. However, as the oil supply tends to be increasingly short, the costs of these two olefin products increase year after year, and their outputs can hardly be guaranteed. Hence, there exists an urgent desire for development of a new process which uses a substitute raw material (e.g. an industrial raw material derived from coal or natural gas) and replaces the old process in which oil is used as the raw material. It is also desired to gain great progress with respect to cost and yield, etc.
Owing to the large-scale development of natural gas and coal chemical industry, as well as the scale-up of the equipment for methanol production, methanol becomes the most promising non-oil resource. As a result, the industrial utilization of methanol becomes a common focus of both academia and industry. As a new process with high additional value which may replace the old process using oil resources, the preparation of olefins from methanol has tremendous market value and application prospect.
A lot of research work has been done previously in this area. However, no satisfactory research results have been obtained to date. For example, Patent CN1166478 describes a method of preparing low carbon olefins such as ethylene and propylene from methanol or dimethyl ether, wherein the catalyst used is a SAPO-34 molecular sieve catalyst, the reaction is conducted in a dense-phase bed circulating fluidized reactor, and ethylene and propylene are the main olefin products at a reaction temperature of 500-570° C., a reaction pressure of 0.01-0.05 MPa and a space velocity of 2-6 h−1. Because the price of ethylene is low, the additional value of the whole process is rather low. Additionally, the high reaction temperature in this process deactivates the catalyst rapidly, and thus the catalyst needs to be replaced or regenerated frequently, leading to linear increase in operating cost.
Therefore, it will be desirable to develop a new process for preparing olefins by using methanol as a raw material, which process has high selectivity to propylene and C4 olefin, and decreased selectivity to ethylene and C6 or higher olefins having low additional value.